1. Field of the Invention
The present invention relates to a pneumatic radial tire for motocross, and more specifically to an improvement in belt durability of the pneumatic radial tire for a motocross which travels on unleveled rough ground.
2. Description of the Prior Art
In motocross tires for traveling on unleveled ground, the tire carcass is, in general, of bias structure. In the bias structure carcass, as is well known, the carcass is formed by arranging cords constituting each carcass ply at an inclination angle of 30 to 45 degrees with respect to the tire circumferential direction in such a way that the arranged cords of two adjoining plies are laid one upon another and intersect each other. However, since an excellent traction performance (adhesive friction of a tire on a ground) is strongly required for the motocross tire, recently radial tires provided with excellent traction performance as compared with the bias-structure tires have been put to the practical use.
In the radial tire for motocross, however, it is impossible to increase the bending stiffness at the tire crown portion, because of the peculiarity such that the tire travels on a greatly uneven ground. Therefore, textile cords are preferably used as a belt reinforcing material arranged at the crown portion; and the belt is formed by arranging cords at an inclination angle (e.g. 20 to 35 degrees) smaller than that in the ordinary radial tire with respect to the tire circumferential direction in the same way that the arranged cords of usually two layers are laid one upon another and intersect each other.
In prior art radial tires, however, there still exists a problem with belt durability such that cracks will readily be produced only on one side of the layers due to the directional relationship between the inclination direction of cords of the outermost belt-constituting layer and the rotational direction of the tire. This is because the belt is formed between the carcass and the tread so as to cover the entire tread extending along the round carcass profile in cross section, so that only the central portion of the tread is in contact with ground in the ordinary travel and therefore a peculiar stress is generated at one end of the belt layer in the case of the motocross tire, thus resulting in crack failure.